Geographic Range

White ibises (Eudocimus albus) are wading birds that are native to the coastal regions of southeastern North American. Year-round, their range extends from Delaware south to the shores of Central America. During the end of the summer months, however, white ibises can be found as far north as New York. During the spring breeding season, their population seems to be localized on the southern portion of the Atlantic coast (south of Virginia) including the entire U.S. and Mexico Gulf coast, Cuba, Dominican Republic, the Brazilian and Colombian coast, and Hispaniola. Remote breeding locations can also be found on the northern coast of Venezuela. During the winter months, white ibises migrate south of the Carolinas and localize in the coastal areas of the United States and internationally. (Frederick and Ogden, 1997; Frederick, et al., 1996; Frederick, 1987; Heath, et al., 2009)

Habitat

Because they require shallow water for feeding, many white ibises are found in coastal, aquatic environments such as ponds and lakes or inland wetlands. White ibises are primarily found in coastal regions during the summer and winter months and move inland towards the beginning of the breeding and nesting seasons. Because water depth is of the upmost importance for their feeding and reproductive behaviors, white ibises may shift locations due to the rise and fall of water levels. For example, white ibises have been known to move inland to fields and wetlands when floodwaters advance, exceeding depths of 20 cm. (Bildstein, et al., 1990; Heath, et al., 2009)

Physical Description

White ibises are white, long-legged wading birds with black wing tips. They possess a long curved bill that aids in feeding. A sexually dimorphic species, adult males are significantly larger than adult females, weighing, on average, about 1,050 grams compared to 750 grams among female ibises. Female bills and wingspans are, on average, shorter than males. The wingspan of white ibises ranges from 56 to 68 cm. Adolescent white ibises lack feathers until they are 4 to 5 days old. At this point, feathers are brownish gray, and do not become completely white until adulthood. Upon hatching, juvenile white ibises exhibit a pinkish white bill that is relatively straight. After the first two weeks, the bill begins to elongate, curve downward, and appears similar to the long distinctive adult bill. During the breeding seasons, male and female white ibises display sexually monomorphic changes. This includes the pink coloration of the bill that fades and becomes black at the tip later in the breeding season. Also, the legs and faces of breeding white ibises are red. A red gular pouch is also displayed by female white ibises. This structural feature is present for about 10 days during the displaying phase of breeding. (Bildstein, 1987; De Santo, et al., 1990; Heath and Frederick, 2006; Heath, et al., 2003; Heath, et al., 2009; Kushlan-A, 1977; Petit and Bildstein, 1986)

Reproduction

White ibises are a monogamous species that possess five stages during the breeding season, which begins in early spring (March). The five stages include display, copulation, egg-production, incubation, and chick rearing. These stages are tied to major behavioral changes exhibited by both sexes. The first stage is display, which lasts about 10 days. This is when mate choice is decided and display flights take place. Involving many individuals of the colony, these display flights result in the birds flying in a circular motion around the perimeter of the colony, diving up and down. Mate choice seems to be catalyzed by the formation of a red gular sac on the female. The females display these bright red pouches to the males, which initiates the pair formation. Once pairing has occurred, the male and female lock heads by wrapping their necks together. The pair then proceeds to lunge their heads toward the ground in a thrust-like motion. Males take the process of pairing extremely serious and have been known to attack, mate by force, or abandon the nest if another male has breed with his mate. However, some studies emphasize that this forced copulation behavior has not been directly observed, so there is some question about this tactic. (Frederick, 1987; Heath and Frederick, 2006; Heath, et al., 2009; Kushlan-C, 1977)

The second step is the copulation stage, when mating occurs. Males display many courtship behaviors including rubbing their bill on the female’s head, preening, and head-nodding at the female. Males may become violent during this period and sometimes injure the females. Due to the aggressive nature of displaying males, females approach in a submissive manner and produce a high-pitched squeal as a greeting. During this time, nest construction begins. Nest construction is dependent on both parents. Female white ibises construct the nest by grabbing twigs with their bill and forcing them into the nest framework, while males collect dead or living twigs for the females to use. Males may steal twigs and other wooden materials from other nests. Nests are normally located in the tops of trees, specifically in the crook of the trunk. However, nests have also been located on the ground. After the nest construction period, the third stage of breeding, egg production, begins. Males are still extremely aggressive towards other males of the colony and to their own mate, as a way of protecting their young. The last stages are egg incubation, which lasts 3 weeks and chick-rearing, which lasts 6 weeks. (Frederick, 1987; Heath and Frederick, 2005; Heath and Frederick, 2006; Heath, et al., 2003; Heath, et al., 2009; Kushlan-C, 1977)

White ibises breed once a year during the spring months, from March to April. During the egg production stage, which occurs 4 to 5 days after mating, female ibises lay one egg every other day, with an average clutch size ranging from 2 to 4 eggs. In one rare instance, a clutch size as large as 7 eggs was recorded. The clutch size can vary depending on geographic location. For example, inland populations tend to have larger clutches of 2 to 3 eggs, compared to coastal populations that average 1 to 2 eggs. Each clutch is concluded within a week. The eggs can be brown or cream, with blueish green spots. On average, eggs weigh about 50.8 g and measure 58 by 39 mm. Once the eggs are hatched, both parents contribute to the warmth of their young by laying on top of them, which last about 3 weeks. Sexual maturity for both males and females is reached at age 3. (Frederick, 1987; Heath and Frederick, 2006; Heath, et al., 2009)

During the incubation period, female white ibises leave to gather food for the young, while the male white ibises stay to protect the nest. The incubation period, which is primarily the male's job, lasts about 3 weeks. Because males are significantly bigger and more aggressive than females, this likely provides better protection for the hatchlings. Studies have shown that when male white ibises are incubating and caring for their young, testosterone levels drop considerably. Some hypothesize this enhances their ability to stay and watch over the young. Hatchlings are initially unable to move due to their weak legs. Additionally, young white ibises are unable to see for 1 to 3 days, because their eyes have not yet opened and do not fully open until day 9. Due to these limitations, the young are completely dependent upon their parents. Both of the parents aid in the feeding process. Because young white ibises have underdeveloped neck muscles, hatchlings are unable to sustain the weight of their neck and have to lie on their side for 1 to 2 days until their muscles properly develop. During this time, parental ibises lie on top of their young to ensure warmth. On their 2nd day, adolescent ibises begin eating food. The young remain in the nest for about 40 days, while completing short flights and gaining the ability to find food. Adult ibises teach their young to fly by standing close to them and beginning to take off. This allows the young to mimic the adults’ technique. Once they are able to fly, juveniles are independent and depart from the original colony. (Frederick, 1987; Heath and Frederick, 2005; Heath and Frederick, 2006; Heath, et al., 2003; Heath, et al., 2009)

Lifespan/Longevity

From banding data, the oldest white ibis found in the wild was 16 years and 4 months old. However, the average lifespan and survivorship of wild adult ibises has not been reported. The maximum lifespan observed in captivity was 27 years and 7 months, with an average in captivity of about 20 years. Nestling survivorship is considerably lower after the first 20 days post-hatching and varies significantly with age and year, which may be explained by predation. For example, in the coastal regions of North Carolina (1980 to 1982) the survivorship of nestlings to 10 days old was 89% in the first year, 86% in the second year, and 68% in the third year. In the Everglades, nestling data was collected from the same colony for 21 days over three years. To day 21 in the Everglades, nestling survivorship ranged from 37 to 83%. In addition, nestling survivorship for coastal Louisiana is 55%. In Florida, nestling survivorship to 60 and 90 days old was 59 to 94% and 44 to 78%, respectively. (Brouwer, et al., 1993; Clapp, et al., 1982; Heath, et al., 2009)

Behavior

White ibises practice a colonial lifestyle. These birds are highly sociable and are found in large colonies of 500 to 15,000 birds at all times. The large colonies aid in their protection from possible predators. For example, if a predator threatens colony members, large groups of ibises will come together to alleviate the threat. The colony participates in many activities together including feeding, foraging, nesting, mating, and roosting. (Heath, et al., 2009)

While searching for food, white ibises forage primary through tactile probing, by thrusting their bill into the wetland substrate, while traveling at a very slow pace. Once prey are found and removed from the substrate, white ibises swallow the prey by rapidly lunging their head forward. White ibises feed in large groups (>5,000 birds) primarily in the early morning and/or late evening. However, this early morning and late evening feeding is not observed during the nesting period. (Frederick and Ogden, 1997; Heath, et al., 2009; Kushlan, 1979)

These birds may fly over 30 km in a single trip. While in flight, these birds remain in “V”-shaped groups known as skeins. This V formation probably increases aerodynamics while in flight. Learning to fly in this formation is essential to the survival of adolescent white ibises. Juveniles that are unable to develop this behavioral pattern exhibit a significantly higher mortality rate. (Heath, et al., 2009; Petit and Bildstein, 1986)

Home Range

White ibises tend to travel based on the abundance of food and conditions of the current habitat. Foraging flights regularly take place and are usually less than 40 km at one time. Colonies of greater than 1,000 breeding pairs cover over 800 square kilometers of wetlands. (De Santo, et al., 1997; Frederick and Ogden, 1997; Heath, et al., 2009)

Communication and Perception

White ibises locate prey by non-visual, tactile sensations. They are relatively quiet birds, but are known to communicate with a "honking" noise to others in the colony. Newly-hatched ibises have a distinctive chirping or "begging" call. This verbal communication of adolescent ibises enables the parents to distinguish their young from the rest of the colony. During the breeding season, male and female ibises develop distinct coloration, which is a form of nonverbal communication, indicating the beginning of courtship behavior. During the time of confrontations, males exhibit visual communication patterns as warning signals including bill chattering and horizontal thrusting toward opponents. (De Santo, et al., 1990; Heath and Frederick, 2006; Heath, et al., 2003; Heath, et al., 2009)

Food Habits

White ibises are foraging birds that search for their prey in shallow waters, about 10 to 15 cm deep. Their diet includes insects, crustaceans, small fish, snails, frogs, and other aquatic organisms. White ibises hunt by non-visual, tactile clues provided by their long, curved bill. This long bill allows them to dig into the mud and find the prey. Their long bill also helps search for food at slightly deeper water levels. However, food availability is extremely limited by the water levels. For example, if water levels are too high, white ibises cannot forage the wetland substrate their prey inhabits. In contrast, when water levels are too low, many of the aquatic organisms they consume are no longer present. Adult white ibises feed in very large groups, while juveniles feed on the perimeter of the colony in the less desirable waters. No diet variation has been observed between juveniles and adults. Because males are larger, they forage longer than females to meet energy requirements. (Aguilera, et al., 1993; Bilstein, 1983; Heath and Frederick, 2006; Heath, et al., 2009; Kushlan, 1979)

Predation

White ibises have few predators, such as fish crows, which consume ibis eggs. To combat this predation, parental white ibises rarely leave the nest unattended. Males display intense aggressive behaviors to fend off fish crows. However, when fish crows discover an unattended nest, they will consume all eggs in the nest. Predation by these crows decreases the colony's nesting success and is a major cause of mortality experienced by white ibises. White ibis eggs are also preyed on by black-crowned night herons, Virginia opossums, raccoons, and rat snakes. Adults have few predators. Alligators have been thought to induce a predatory stress on white ibises, but no studies substantiate this hypothesis. (Dronen and Blend, 2008; Heath, et al., 2009; Shield and Pamell, 1986)

Ecosystem Roles

Colonies of white ibises affect local ecosystems by taking up nutrients while foraging and transporting them to nesting areas. This allows nutrients, such as potassium, phosphorus, and calcium, to be widely dispersed throughout the habitat. Many parasitic organisms inhabit adult white ibises including 17 species of nematodes, 21 species of trematodes, 2 species of cestodes, 2 species of acanthocephalans, 3 species of mites, and 5 species of lice. (Forrester and Spalding, 2003; Heath, et al., 2009)

Economic Importance for Humans: Positive

In the past, white ibises had been hunted and sold as a food source by Native Americans and individuals that lived in close proximity to their colonies. In some areas, this practice still continues today. Some consider this bird to be a delicacy due to their appealing taste, which is thought to be derived from their crayfish diet. (Audubon, 1835; Heath, et al., 2009)

Economic Importance for Humans: Negative

In Louisiana, crayfish producers have been significantly impacted by crayfish predation from white ibises. Crayfish farmers have been known to hunt ibises to protect the crayfish from predation. White ibises have no other known adverse effects on humans. (Heath, et al., 2009)

Negative Impacts

crop pest

Conservation Status

According to the IUCN red list, white ibises are listed as least concern and have no special status on the US Migratory Bird Act and the US Federal List. Currently, their population is large and relatively stable. However, according to the North American Waterbird Conservation Plan (NAWCP), their population is of moderate concern due to their migratory pattern and their limited range during non-breeding and breeding seasons. Deterioration of their natural habitat is a big threat to their population. For example, in the 1930s to 1970s there was a significant decline (>95%) of the white ibis population in the Florida Everglades. This may have been in part caused by human involvement that interrupted the natural hydrologic regime of the Everglades, which caused a decrease in food availability. In addition, toxic contaminants in the wetlands, such as mercury, also threatened their population. White ibises captured from the Florida Everglades in 1999 to 2001 had an accumulation of mercury in their blood, which has detrimental effects on their nervous and reproductive systems. Steps to restore and reduce wetland contamination could significantly improve population sizes, including protection of foraging locations and preservation of the natural hydrologic regime. (Frederick, et al., 1996; Heath and Frederick, 2005; Heath, et al., 2009; Kushlan, et al., 2002)

Contributors

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

Neotropical

living in the southern part of the New World. In other words, Central and South America.

acoustic

uses sound to communicate

agricultural

living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

diurnal

active during the day, 2. lasting for one day.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

fertilization

union of egg and spermatozoan

filter-feeding

a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.

food

A substance that provides both nutrients and energy to a living thing.

freshwater

mainly lives in water that is not salty.

insectivore

An animal that eats mainly insects or spiders.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

marsh

marshes are wetland areas often dominated by grasses and reeds.

migratory

makes seasonal movements between breeding and wintering grounds

molluscivore

eats mollusks, members of Phylum Mollusca

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

polymorphic

"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic.
Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

soil aeration

digs and breaks up soil so air and water can get in

tactile

uses touch to communicate

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

Audubon, J. 1835. Ornothological Biography, Or, An Account of the Habits of the Birds of the United States of America: Accompanied by Descriptions of the Objects Represented in the Work Entitled, The Birds of America, and Interspersed with Delineations of American Scenery and Manners, Volume 3. Edinburgh, Scotland: A. Black.

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The Animal Diversity Web is an educational resource written largely by and for college students. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control.

This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.